1. Field of Applicable Technology
The present invention relates to a circuit for cancelling adjacent-track crosstalk that is contained in a playback chrominance signal produced by a video tape recorder (hereinafter referred to as VTR). More specifically, the invention relates to a circuit for cancelling crosstalk in a playback down-converted chrominance signal which is to be supplied directly to another VTR to execute re-recording of a video signal.
2. Prior Art Technology
In a color composite video signal, luminance information is conveyed by a luminance signal of varying amplitude which is generally referred to as the Y signal, and color information is conveyed by a signal (the C signal) which is variously referred to as the chroma signal, the chrominance signal, or the carrier chrominance signal. To avoid confusion in the following, that signal will be referred to as the carrier chrominance signal throughout. The carrier chrominance signal is derived by simultaneous phase and amplitude modulation of a carrier whose frequency (e.g. 3.58 MHz) is usually selected to be higher than the maximum utilized frequency in the luminance signal frequency band. In general, with a domestic-use color VTR, a method of recording is utilized whereby the luminance signal and carrier chrominance signal components of a composite video signal are separated from the composite video signal, the luminance signal is used to frequency modulate a high frequency carrier to obtain a wide-band FM luminance signal, and frequency down-conversion of the carrier chrominance signal is executed to obtain a down-converted chrominance signal which occupies a frequency band that is lower than that of the FM luminance signal. The FM luminance signal and down-converted chrominance signal are then combined to obtain a recording signal, which is recorded on magnetic tape. In order to maximize the tape utilization efficiency with such a domestic-use VTR and thereby increase the maximum possible recording time, a "guard band-less" method of recording is usually employed, whereby no guard bands are provided between mutually adjacent recording tracks on the magnetic tape. As a result, crosstalk is induced into the contents of each track, from the two mutually adjacent tracks which are positioned on each side of the track, which results in inteference being produced on a display image when playback of the tape is executed. To alleviate this problem, at least two magnetic heads are used during both recording and playback for respectively recording and reproducing alternate tracks, with the respective azimuth angles of the head gaps of these magnetic heads being mutually different by a specific amount (e.g. .+-.6.degree.). This method of crosstalk reduction is very effective for the FM luminance signal, and in particular for the high frequency components of that FM luminance signal. However it is much less effective for the down-converted chrominance signal. Thus, the down-converted chrominance signal that is obtained by playback of a magnetic tape using such a VTR will contain substantial levels of crosstalk.
To overcome this problem, a method has been proposed in the prior art which is known as the PS (Phase Shift) method, whereby the phase of the carrier chrominance signal (and hence the down-converted chrominance signal) is shifted by 90.degree. at the start of each horizontal scanning interval prior to recording, and whereby this successive 90.degree. phase shifting applied to successive horizontal scanning intervals (i.e. successive 1 H intervals, where H denotes a horizontal scanning interval) is executed in a fixed direction during recording of one track on the magnetic tape and in the opposite direction during recording of the succeeding track, then in the first direction, and so on, i.e. with the direction of phase shift alternating for successive tracks on the tape.
FIGS. 1A to 1E are conceptual diagrams for illustrating this PS method. In the following description and also in subsequent descriptions of embodiments of the present invention it will be assumed that a VTR having two magnetic heads is utilized. However the invention is equally applicable to a VTR having a greater number of magnetic heads. In FIGS. 1A to 1E, the directions of the arrows shown in full-line form indicate the phase of the down-converted chrominance signal that is recorded on the magnetic tape, while the directions of the arrows shown in broken-line form indicate the phase of crosstalk that is induced during each 1 H interval, from adjacent tracks. FIG. 1A shows the 90.degree. phase shifts which occur in successive 1 H intervals for the down-converted chrominance signal of the recording signal that is applied to one of the magnetic heads of the VTR during recording of one track on the magnetic tape. Similarly, FIG. 1B shows the 90.degree. phase shifts which occur in successive 1 H intervals for the down-converted chrominance signal of the recording signal that is applied to the other one of the magnetic heads of the VTR during recording of the succeeding track on the magnetic tape. FIG. 1C shows the phase relationships in a chrominance signal (e.g. a playback carrier chrominance signal obtained by frequency conversion of a playback down-converted chrominance signal) that is subsequently obtained from the track recorded by the signal of FIG. 1A, after the playback signal has been processed to restore the phase of the signal during each 1 H interval to the original condition, i.e. to the condition prior to executing the aforementioned 90.degree. phase shift processing at the time of recording. FIG. 1D shows the carrier chrominance signal that results from delaying the signal of FIG. 1C by 1 H (i.e. one horizontal scanning interval). As can be understood from FIGS. 1C and 1D, the phase of the desired carrier chrominance signal is identical for both of the signals shown, during each horizontal scanning interval, while the phase of the respective crosstalk components contained in the signals shows a difference of 180.degree. during each horizontal scanning interval. If it is assumed that there will usually be a strong degree of vertical correlation in the chrominance information in successive 1 H intervals (i.e. in successive horizontal scan lines represented during these horizontal scanning intervals), then it is possible to mutually add the signals of FIGS. 1C and 1D, to obtain the signal shown in FIG. 1E. In this signal, the crosstalk components of the playback chrominance signal have been mutually cancelled.
As described above, a color composite video signal is recorded by converting the luminance and chrominance components of that signal into an FM luminance signal and a down-converted chrominance signal. At the time of playback, the FM luminance signal is demodulated to recover the original luminance signal, while the down-converted chrominance signal is subjected to frequency up-conversion to recover the original carrier chrominance signal. If the PS method of crosstalk cancellation described above is being utilized, then the carrier chrominance signal which has thus been obtained is processed as described referring to FIGS. 1A to 1E, and the resultant carrier chrominance signal with crosstalk excluded is combined with the recovered luminance signal, to obtain the original color composite video signal (with treatment of the synchronizing components having being omitted from this description, for simplicity).
In the prior art, re-recording (sometimes referred to as video dubbing) of a composite video signal that has thus been recovered by playback of a magnetic tape is generally executed by supplying the recovered composite video signal to another VTR, to be recorded thereby. However the playback down-converted chrominance signal obtained from the magnetic tape is first passed through a band pass filter, then is frequency up-converted by a frequency converter circuit to recover the carrier chrominance signal. The frequency conversion operation is controlled by APC (Automatic Phase Control) and AFC (Automatic Frequency Control) systems. Due to transfer through the band pass filter, the phase/frequency characteristic of the playback chrominance signal is adversely affected, and the chrominance signal is also adversely affected by the action of the AFC and APC systems, which introduce increased levels of phase modulation noise. Thus, when this carrier chrominance signal is then again down-converted and re-recorded (in combination with the FM luminance signal) in another VTR, and the resultant recorded tape is subsequently played back, there will be a significant deterioration in the quality of the video image that is obtained, by comparison with that provided by the playback video signal which was used for this re-recording. If a tape which has thus been re-recorded is then re-recorded onto another tape, this deterioration of image quality will further increase. This places a sharp limitation upon the number of times that such re-recording can be successively executed.
There are some types of tape re-recording apparatus in which the playback down-converted chrominance signal obtained from one magnetic tape is directly recorded onto another tape, without first executing frequency up-conversion then subsequent down-conversion of that down-converted chrominance signal. However with such a prior art apparatus, crosstalk cancellation is not executed for the playback down-converted chrominance signal prior to re-recording. Thus, when playback is executed of a tape that has been re-recorded in this way, there will be a substantial level of crosstalk noise contained in the playback down-converted chrominance signal. This has a very adverse effect upon subsequent processing that is applied to recover the carrier chrominance signal from that down-converted chrominance signal, causing a severe deterioration of image quality.